Certain polymeric materials are capable of being polarized. In general, it has been found in the past that a polymeric material such as poly(vinylidene fluoride) can be polarized as by stretching a film at a temperature below its melting point, such as at room temperature or at a temperature of about 70.degree. C. Stretching can be on an uniaxial basis or a biaxial basis and often the film is stretched to increase the length by two or three times. The film then is subjected to a DC field of at least 1 MV/cm. Poly(vinylidene fluoride) is a common and a preferred material for polarization, since it has been found to have a high capability of polarization response, thereby providing desirable piezoelectric properties. Subjecting such a stretched film, for example, using an appropriate DC field applied in a direction perpendicular to the plane of the stretched field causes an orientation of the molecular dipoles of the polymeric materials. In case of the poly(vinylidene fluoride), fluorine groups have a negative charge and the hydrogen atoms attached to the other carbon of the vinylidene fluoride unit of the polymer have a positive charge. Vinylidene fluoride units in a poly(vinylidene fluoride) can exist in at least two different crystalline forms, in a planar zig-zag polar form or trans form (beta form or Form 1) or a non-polar and non-planar form referred to as alpha form or Form 2.
Many other polymeric materials can be polarized to provide piezoelectric films in a like manner. It is desired to provide piezoelectric polymeric film materials which provide improved ferro- and piezoelectric activities which are improved over those of the individual polymeric materials, especially at elevated temperatures.
Such polarized materials have a number of uses, including use in the making of transducers, push button onoff switches and many other uses which utilize ferro- or piezoelectric properties of such polarized materials.
It would be highly desired to have improved piezoelectric polymeric films which have improved ferro- or piezoelectric properties. Likewise, it would be highly desired to provide improved process for making such improved piezoelectric film materials.